This invention relates to a process and an apparatus for the production of soluble instant coffee in powder form.
Soluble coffee powder is conventionally produced by freeze-drying or spray-drying after evaporation of a coffee extract obtained by the percolation of an extraction liquid through cells filled with ground roasted coffee (Sivetz, Coffee Processing Technology, Volume 1, pages 262, 263, AVI, 1963).
Extraction is carried out in countercurrent fashion, i.e., the water under pressure at a temperature of 150.degree. to 180.degree. C. is introduced into the cell containing the batch of ground roasted coffee which has been most intensively extracted (having undergone N extractions) at the bottom of that, and then the liquid extract of this extraction cell is passed through the extraction cell containing the batch of coffee which has been extracted (N-1) times, and so, on until the liquid extract passes through the cell which has just been filled with fresh, ground roasted coffee and the final extract leaves the last cell at a temperature on the order of 100.degree. C.
In such countercurrent extraction, the most intensively extracted coffee is thus subjected to the highest temperature while the fresh coffee is subjected to the lowest temperature.
A distinction is normally drawn between the hot cells, which contain the most intensively extracted coffee, and the cold cells, which contain the least intensively extracted coffee.
After each extraction cycle, the cell containing the most intensively extracted coffee is emptied, filled with fresh coffee and, after the cells have been suitably interconnected, another extraction cycle begins.
It is known that, the higher the extraction temperature, the higher the yield. On the other hand, however, the use of a high temperature results in hydrolysis phenomena which produce organoleptically disagreeable compounds.
Thus, the extract produced in the hot cells is typically charged with compounds which give rise in the final extract to the formation of a soluble coffee powder which, after redissolution in hot water, has a marked taste of hydrolysis and caramel.
It has thus been proposed, as for example in U.S. Pat. No. 4,129,665, to extract the coffee by partly evaporating the extract from the hot cells before it is reintroduced into the cold cells, the evaporated extract having a temperature above 120.degree. C. More particularly, evaporation is carried out in an expansion evaporator consisting of a pressure reducing unit followed by a flash evaporator.
In addition, in this process, at least as much extraction liquid as that which has been evaporated is added to the extract which has undergone evaporation.
In this process, an improvement in the end product is clearly obtained because the tastes of hydrolysis and caramel are distinctly reduced.
In spite of this result, however, there is a simultaneous reduction in the strength of the organoleptically desirable compounds, which is all the more marked if the treated coffee has been weakly roasted.
One way of overcoming this disadvantage may be to reduce the effect of the process by increasing the flash evaporation temperature although, in this case, the compounds to be suppressed are less effectively eliminated.
Accordingly, the object of the process according to the invention is to enable the organoleptically troublesome compounds to be eliminated without a simultaneous reduction in taste.
In addition, in the prior art, solids, such as polysaccharides or proteins, are also present in suspension, even after filtration of the final extract, which tends to eliminate the solid particles which may have been entrained from the coffee suspension, and have to be eliminated to enable a coffee powder which dissolves perfectly without any solids appearing in the cup to be obtained after evaporation and freeze-drying or spray-drying of the extract.
The suspended solids are normally eliminated by centrifugation, the sediment obtained then being decanted, the supernatant decantation liquid being reintroduced into the final filtered extract while the solid residue obtained is eliminated.
The main disadvantage of this process is that it produces a sediment which has to be retreated by decantation and which is not easy to handle.
In addition, the suspended solids cannot always be satisfactorily eliminated by centrifugation.
Accordingly, another object of the present invention is to provide a process which in addition, reduces the insoluble fraction present in the final extract.